Metallurgically bonded wear resistant texture coatings for aluminum alloys and metal matrix composite electrode for producing same

ABSTRACT

A coating process using a welding electrode comprises the steps of: electro-spark depositing a composite on to a substrate, wherein the composite consists essentially of alumina particles in a matrix, wherein the matrix is a metal selected from the group consisting of aluminum or an aluminum alloy. The substrate herein is an aluminum alloy. The coatings involve only aluminum and aluminum compounds, thereby intermetallic compound formation and galvanic corrosion of the coating-substrate materials system is avoided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 15/187,921, filed Jun. 21, 2016, which claimed benefitestablished by provisional application Ser. No. 62/197,281, filed Jul.27, 2015, the contents of which are incorporated herein by reference.

BACKGROUND Field of the Invention

The present application relates to protective coatings for metal alloys.Specifically, a metal matrix composite consisting essentially of analuminum alloy and a hard aluminum compound is electro-spark depositedon a substrate, thereby metallurgically bonded to any aluminum alloysubstrate.

Description of the Related Art

Coating aluminum alloys without impacting the alloy properties isimpossible using typical techniques. State of the art coating processesare not suitable for aluminum alloys because of the following problemsrelated to application of commercially available protective coatings onAl alloys. First, aluminum engineering alloys are often age hardened,and so, any thermal cycle following controlled aging (precipitation heattreatment) can cause over aging, that is, it will weaken the alloy.Conventional coating systems (examples include thermal spray, diffusionprocesses, cathodic arc PVD, electron beam PVD, cold spray among others)require a post coat thermal cycle to achieve a metallurgical bond (adiffusion bond). Hence, a metallurgical bond cannot be obtained betweenconventional coating systems and an aluminum alloy without damaging themechanical properties of the substrate. Second, aluminum formsintermetallic compounds (hard brittle materials) with nearly all otherknown metals (including all metals in conventional coating systems), andso, any thermal process intended to form a diffusion bond between thecoating and the aluminum alloy substrate will result in intermetalliccompound formation. The formation of brittle layers in a coating cancause poor mechanical properties, that is, cracking and spalling of thecoating. Third, galvanic corrosion can occur whenever different metalsin electrical contact are exposed to an aggressive environment. Nearlyall components of state of the art protective coating systems will forma galvanic couple with aluminum or aluminum alloys, so rapid corrosiondegradation will occur whenever the state of the art materials system(substrate and coating) is exposed to an aggressive environment.Consequently, the most successful coatings for aluminum alloys aremechanically bonded overlay products (primarily thermal spray), but, dueto the problems discussed (poor bond strength and galvanic corrosion),these products have limited service life.

Needed then is a metal matrix composite electrode for electro sparkdeposition (ESD) which can make metallurgical bonded coatings onaluminum alloys without any of the problems encountered with state ofthe art coating systems.

SUMMARY

The invention comprehends a protective coating, a metal matrix composite(MMC) electrode for producing same and a coating process using the MMCwelding electrode. The protective coating for aluminum alloys comprisesa composite consisting essentially of alumina particles in a matrix,wherein the matrix is a metal selected from the group consisting ofaluminum or an aluminum alloy. In the preferred embodiment the aluminaparticles consist of aluminum oxide in powder form. The coating can betextured by deposition of aluminum protrusions on the substrate(aluminum alloy electrode) or textured with additives (aluminum plusaluminum oxide electrodes) and both types of textured coatings canfurther be hard coated (hard anodized).

The welding electrode to produce the protective coating comprises ametal matrix composite consisting essentially of alumina particles in amatrix, wherein the matrix is a metal selected from the group consistingof aluminum or an aluminum alloy. As above, the alumina particlesconsist of aluminum oxide in powder form.

The coating process using the welding electrode above comprises thesteps of: electro-spark depositing a composite on to a substrate,wherein the composite consists essentially of alumina particles in amatrix, wherein the matrix is a metal selected from the group consistingof aluminum or an aluminum alloy. The substrate herein is an aluminumalloy. In an alternative embodiment, the coating process compriseselectro-spark depositing aluminum on an aluminum alloy to form atextured surface; and, hard anodizing the textured surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows block on ring wear test results of alloy 6061 and of alloy6061 with the instant coating under particular test conditions.

FIG. 2 shows immersion corrosion test results of alloy 6061 coupons withinstant coatings (C-22 and CZ), a prior art texture coating for aluminumalloys and a prior art wear resistant coating under particular testconditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Electro Spark Deposition (ESD) is a pulsed micro-welding process thatcauses insignificant heating of the substrate during coating deposition,and so, it can be used to make metallurgically bonded (welded) coatingson aluminum alloys without over aging them. To avoid intermetalliccompound formation, an ESD electrode consisting of an aluminum oraluminum alloy matrix composite with an aluminum oxide particledispersion (a metal matrix composite or MMC) is utilized herein. Thecoating produced by ESD with the instant MMC electrode has aluminaparticles in an aluminum or aluminum alloy matrix. Since the coatingdeposit contains only aluminum and aluminum compounds, the formation ofintermetallic compounds between the coating and the substrate is notpossible. The coating has a bond strength similar to alloy tensilestrength (>13,000 psi) of the substrate compared to mechanically bondedproducts with typical bond strength of about 2000 psi for othercommercially available coatings. The coating preferably has a thicknessin a range of 10 μm to 100 μm. In addition, since the coating depositcontains only aluminum and aluminum compounds, galvanic corrosion of thematerial system (coating and substrate) in service is prevented.Corrosion resistance provided is far superior to commercially availablestate of the art wear and/or texture coatings for aluminum alloys.

More particularly, the present protective coating for aluminum alloyscomprises a composite consisting essentially of alumina particles in amatrix, wherein the matrix is a metal selected from the group consistingof aluminum or an aluminum alloy. Although the particle size, shape andsurface area of ‘particles’ may vary, that is, powder, whiskers orfibers among others, in the preferred embodiment alumina particles asused herein means aluminum oxide in powder form.

The substrate surface can be textured by aluminum deposition or texturedby deposition of aluminum and additives (MMC electrodes) and either typedeposit can further be hard coated (hard anodized). Surface finishes mayvary and range from 100 μ-in to 1400 μ-in for example. Aluminum is arelatively soft material, and so, wear is a significant mode of failurein many applications. In addition, aluminum plates are used in floorsand other applications where a rough surface is needed for traction. Inboth cases, a metallurgically bonded coating (smooth or rough)containing a hard phase or having a top coat is needed to reduce wearand/or increase the coefficient of friction.

Aluminum alloys are used in a variety of industries and commercialapplications, which include but are not limited to ships, airplanes,cars, trucks, trailers and battle tanks. In all cases however, thecoating of the invention would be used to provide superior wearresistance and/or increased coefficient of friction to a component ofthe machine.

Because ESD equipment is used to produce a micro-weld deposit ofaluminum and aluminum oxide on an aluminum alloy substrate, a weldingelectrode of a particular make-up must produce the protective coating.The welding electrode herein to produce the protective coating comprisesa metal matrix composite consisting essentially of alumina particles ina matrix, wherein the matrix is a metal selected from the groupconsisting of aluminum or an aluminum alloy. As above, the preferredalumina particles consist of aluminum oxide in powder form.

The manufacture of aluminum-aluminum oxide metal matrix compositeelectrodes can be accomplished by several techniques, examples include,but are not limed to, metal infiltration molding, extrusion, thermalspray, pressing and sintering powder mixtures and cold spray. Forexample, metal infiltration molding involves injection of liquid metalinto a mold containing solid media (particles, or fiber etc.) to createa metal matrix composite (MMC). ‘Infiltration’ can be accomplished byusing pressure or vacuum to force liquid aluminum alloy into a tubefilled with aluminum oxide powder to create the MMC electrode rod, whichcan thereby be used for electrodes in the coating process. The MMCelectrode and ESD process yield a coating on aluminum alloys withouteither intermetallic compound formation or over aging of the substrate,and the coating of the invention eliminates the possibility of galvaniccorrosion between the coating and the substrate in service.

Electro spark deposition (ESD) is the only coating process that does notcause significant heating of the substrate or require post coat heattreatment to obtain a metallurgical bond, so it is ideally suited tocoating application on aluminum alloys. Accordingly, the instant coatingprocess using the welding electrode above comprises the steps of:electro-spark depositing a composite on to a substrate, wherein thecomposite consists essentially of alumina particles in a matrix, whereinthe matrix is a metal selected from the group consisting of aluminum oran aluminum alloy. The substrate herein is an aluminum alloy. Criticalis that both the aluminum-aluminum oxide MMC electrode and the ESDcoating process are necessary to production of a metallurgically bondedcoating on aluminum alloys without formation of intermetallic compounds,without damage to the mechanical properties of the substrate, andwithout the possibility of galvanic corrosion in service.

In an alternative embodiment, the coating process compriseselectro-spark depositing aluminum on an aluminum alloy to form atextured surface; and, hard anodizing the textured surface. Thisapproach yields a metallurgically bonded texture coating of aluminum onthe substrate with a continuous layer of aluminum oxide over the entiresurface. Our data shows this approach increases the coefficient offriction and the wear resistance, plus it should provide improvedcorrosion resistance due to the protective layer of aluminum oxide overthe entire surface.

EXAMPLES

FIG. 1 shows ASTM G-77 Block on Ring wear test results for a 6061 alloyblock and for 6061 alloy blocks with the instant coatings (CZ and C22)under particular test conditions. Sliding wear test results demonstratethat both the coatings of the invention have better wear resistance thanalloy 6061, the aluminum plus aluminum oxide particulates (C-22 coating)has wear resistance twelve (12×) times better than uncoated alloy 6061at 3.6 Kg load and the hard anodized aluminum coating (CZ) has wearresistance 1.7× better than alloy 6061 at 3.6 Kg load.

FIG. 2 shows Immersion Corrosion Test results from alloy 6061 couponswith instant coatings (C-22 and CZ), a prior art texture coating foraluminum alloys and a prior art wear resistant coating under particulartest conditions. Immersion corrosion test results indicate: a) the hardanodized aluminum coating has the best corrosion resistance of theproducts considered, b) the aluminum plus aluminum oxide particulatecomposite coating exhibited somewhat greater degradation than theanodized coating (1.3× greater weight gain than CZ), and c) the priorart products have significantly worse corrosion resistance than eithercoating of the invention (Product 1 exhibited 6.8× greater weight gainthan CZ and Product 2 exhibited 9.1× greater weight gain than CZ). Boththe prior art coatings are mechanically bonded thermal spray products,and in both cases, composition differences between the coating and thesubstrate are sufficient to cause galvanic corrosion in the test.

1. A coating process, comprising the steps of: using an aluminum-aluminum oxide metal matrix composite electrode as a composite, electro-spark depositing said composite on to an aluminum alloy substrate to form a protective coating, wherein said protective coating consists of alumina particles in a matrix, wherein said matrix is a metal selected from the group consisting of aluminum and an aluminum alloy; and, wherein said protective coating is metallurgically bonded as a continuous layer to an entire surface of said aluminum alloy substrate by electro-spark deposition, and wherein said composite material is formed without intermetallic compound formation between said protective coating and said aluminum alloy substrate.
 2. The coating process of claim 1, further comprising the step of forming a textured surface.
 3. The coating process of claim 2, further comprising the step of hard anodizing the textured surface. 